The invention contemplates a transmission wherein the input shaft carries a transversely extending cross head that rotatably mounts two small pneumatic tires located outboard from the shaft axis. The output shaft carries an endless annular track that is engaged by the tread areas of the tires. As the input shaft rotates the tires are caused to orbit around the track surface. The tires transmit drag forces to the track to thereby impart rotational motion to the output mechanism.
The speed ratio may be changed by adjusting the cross head around its axis. Such adjustment changes the rotational plane of the tires relative to the rotational plane of the track so that the tires have different rolling and skidding actions on the track surface.